1. Field of the Invention
The present invention relates to liquid polymer and polyol compositions, solid crosslinked polymer compositions prepared therefrom, and methods for improving coating properties of films and surface coatings. It also relates to preparing polymers and polyols endcapped with phenolic functionalities.
2. Description of the Related Art
Coating formulations usually contain a number of components. A primary component is resin, which can be natural or synthetic. The resin acts as a polymeric coating binder, or polymeric coating vehicle for the coating formulation. In addition, most coatings require a solvent, and the coating may also contain a wide variety of additives. Further, many coatings also contain a crosslinking agent, which after application of the coating vehicle to a substrate, reacts chemically with the resin during a curing stage to produce a film containing a crosslinked network. The crosslinked network is necessary for production of good film properties. The curing stage can be conducted at ambient conditions ("air-dry system"), or at elevated temperatures ("baked system"). In either case, the solvent is evaporated during the curing stage, resulting in a coating film. A number of properties are important for the coating film, including hardness, flexibility, weather resistance (weatherability), chemical resistance, solvent resistance, corrosion resistance, adhesion to various substrates, impact resistance, and several others. The properties depend on many factors including type, molecular weight, monomer composition, and glass transition temperature (Tg) of the resin: type and amount of the crosslinker; curing conditions; curing catalyst: and additives. Variations of these parameters can be used to create a wide range of differences in film properties to fit requirements for a number of diverse applications. However, it is not always possible to optimize all of the desirable properties simultaneously.
For example, hardness and impact resistance are two desirable characteristics of coatings which are somewhat mutually exclusive since high hardness is usually associated with films having high Tgs. Conversely, high impact resistance is associated with low Tg. This necessitates a trade-off between high hardness and high impact resistance. It is frequently possible to optimize one of these properties, but at the expense of the other.
In European Patent Application No. 0 287 233 filed Mar. 28, 1988, and published Oct. 19, 1988, Jones et al. teaches a method to simultaneously obtain both high hardness and high impact resistance in a coating by employing liquid crystalline (L.C.) polymers. The L.C. polymers are characterized by containing mesogenic groups which impart the L.C. character to the polymer. The mesogenic groups are chemical structures that contain a rigid sequence of at least two, and frequently more, aromatic rings connected in the para position by a covalent bond or by other rigid or semirigid chemical linkages. In addition to the mesogenic groups, the polymers contain conventional polymeric units which are attached to the mesogens via a covalent bond.
Jones formulates these L.C. polymers with suitable crosslinking resins, such as aminoplast resins, to create coating vehicles which, upon curing by baking yield films which have both high hardness and high impact values. The enhanced properties are attributed to the L.C. interaction of the various polymer chains. A mesogen which is frequently used consists of the internal esters of two or more molecules of para-hydroxybenzoic acid (PHBA). This mesogen is connected to a polymeric polyol by esterification of the OH groups of the polyol with the residual carboxyl groups of the mesogen.
The L.C. polymers, while possessing good properties, have several drawbacks. First, the mesogenic groups are usually expensive to synthesize and incorporate into the polymer. For example, multiple PHBA end groups require a large quantity of PHBA and significantly increase the resin price. Second, the synthesis is complicated. In one method, the synthesis is based on the use of expensive and toxic dicyclohexylcarbodimide, which renders this method impractical from a commercial standpoint. Another method is based on direct esterification of PHBA with a polyesterdiol at 230.degree. C. in the presence of para-toluenesulfonic acid (p-TSA). Jones teaches that it is important that the acid catalyst be used and that the temperature be controlled to provide the predominantly L.C. phenolic oligoesters. Polymers produced in accordance with the teachings of Jones, however, result in material with poor color, an unacceptably high loss of PHBA via decarboxylation, and a sizable loss of phthalic acid from the polymer due to anhydride formation. In order to be commercially attractive, it would be very desirable to provide the enhanced properties associated with Jones's L.C. polymers without the above-mentioned attendant problems.
Efforts have been made to incorporate active phenolic functionalities into polymeric coating vehicles to enhance curing characteristics or the properties of the prepared coating. However, the coatings produced in accordance with the prior art are generally inferior or difficult to prepare.
U.S. Pat. Nos. 4,446,302, re. 32,136 and 4,416,965, all three to Sandhu et al., disclose polyesters having recurring units derived from diols and diacids and recurring units derived from p-hydroxybenzoic acid. These polyesters are used in electrographic developer compositions. The polymers disclosed in Sandhu et al. have several disadvantages. The recurring units derived from PHBA are blocks of two or more units of PHBA. Also, the polymers have high molecular weight as evidenced by their high inherent viscosities of about 0.3-0.7, (MW 50,000-200,000). Finally, the polymers are carboxyl terminated since they are made from p-acetoxybenzoic acid.
U.S. Pat. No. 2,979,473 to Heinrich relates to an alkyd formed from a polyacid, a polyol and modifier comprising 30-70 mole % monocarboxylic aromatic acid containing from about 50-100 mole % of 2,4-dimethyl benzoic acid.
U.S. Pat. No. 2,993,873 to Heinrich relates to alkyd resins modified by reaction with hydroxybenzoic acids and cured by ambient or baked cures. In either case, no crosslinking agent is added. Rather, the cure proceeds via the unsaturated site in the alkyd resin and coatings produced therefrom do not include benefits achieved by incorporating a crosslinker.
U.S. Pat. No. 4,543,952 to Shalaby discloses copolymers formed by the polycondensation of PHBA, an acid anhydride and diol. As in the Sandhu et al. and Heinrich patents, however, the polymer produced is not PHBA end-capped, but rather has a random structure.
U.S. Pat. No. 3,836,491 to Taft and U.S. Pat. Nos. 4,343,839, 4,374,181, 4,365,039, and 4,374,167 to Blegen disclose compositions capable of being cured at room temperature with a tertiary amine comprising a phenolic terminated polyester component and a polyisocyanate curing agent. These systems are unstable at room temperature and must be stored in two separate packages which are mixed together immediately prior to application. Taft discloses numerous uncapped prepolymer components which can be reacted with a carboxyphenol (e.g., hydroxybenzoic acid) to give a wide variety of capped hydroxy containing polymers for subsequent reaction.
Taft and Blegen, however, relate to two package polyurethane systems whereby mixing and subsequent reaction of the polymer with a polyisocyanate in the presence of a tertiary amine (basic catalyst) results in a rapidly curable composition (few minutes) at room temperature. Coatings prepared according to this method do not exhibit improved characteristics achieved by baking to cure the coating. Furthermore, in order to avoid direct esterification of hydroxybenzoic acid, Taft resorts to a difficult transesterification of the methyl ester of hydroxybenzoic acid. In order to provide an acceptable conversion, a significant excess (ca. 2 fold) of methylsalicylate, a methyl ester of hydroxybenzoic acid, must therefore be used, requiring an additional vacuum stripping operation at 0.05 mm Hg with heating up to 385.degree. F. to remove the excess methylsalicylate. Even then, about 25% of the methylsalicylate could not be removed. Thus, this makes the product and process disclosed in Taft commercially undesirable and noncompetitive.
U.S. Pat. No. 4,331,782 to Linden discloses a method for making a "phenol-functional polyester polymer". According to this patent, hydroxybenzoic acid is pre-reacted in a first stage with an epoxy compound such as Cardura E (glycidyl ester of neodecanoic acid) to produce an adduct as shown below: ##STR1##
This step protects the carboxylic acid of the PHBA and prevents decarboxylation, and it also creates a more reactive hydroxyl site on the adduct so that subsequent reaction with other components is easier. In a second stage, the adduct is reacted with neopentyl glycol, adipic acid, and isophthalic acid to provide the phenol-functional polyester.
Linden teaches that direct reaction of hydroxybenzoic acid with a polyol for synthesis of a polyester is impractical since degradation of the hydroxybenzoic acid is prevalent. This patent further discloses that advantages achieved includes the ability to synthesize a phenol-functional polymer without subjecting hydroxybenzoic acid to conditions amenable to decarboxylation.
Linden also discloses a method whereby a polyester polymer is prepared which is substantially free of reactive aliphatic hydroxyl groups in order to provide increased pot life of a coating composition prepared therefrom. Reactive aliphatic hydroxyl groups, however, are desirable and even critical in some situations.
U.S. Pat. Nos. 4,267,239 and 4,298,658 to Thankachan et al. disclose alkyd resins containing free hydroxyl groups modified by reaction with PHBA. The modified alkyds are cured via a vapor curing process at room temperature with a di- or polyisocyanate in the presence of an amine vapor. These are also two package systems which must be stored separately. Coatings prepared according to this method, have limited properties because they are not formulated with an amino crosslinking agent and baked at elevated temperature.
Japanese Patent Nos. 52-73929, 52-81342, and 53-42338 relate to powder coating compositions comprising an amino resin and a polyester resin having phenolic hydroxyl groups and having a softening temperature of 40.degree. to 150.degree. C. Japanese Patent Nos. 52-81341 and 53-42341 are similar, but they also incorporate double bonds in the polyester structure to allow a second mode (oxidative) of crosslinking to take place in order to reduce the amount of crosslinking required by the amino crosslinking resin, and, consequently, reduce the amount of amino resin required. However, all of these patents are directed to powder coatings which require that the resin system be a solid under application conditions. Hence, they must have a high softening temperature which equates to a high Tg for the polyester resin. Furthermore, powder coatings are a specialized application technique and are not used extensively. More common application techniques require liquid systems.